The present invention pertains to a planar light source device, and more particularly to a planar light source device utilizes a novel light guide pipe and light reflection sheet that are employed particularly favorably in an illumination optical system, and can be used favorably as a backlight optical system of a liquid crystal display device.
Recently, transmission-type liquid crystal display devices are frequently used as display devices in monitors designed for personal computers and in thin televisions and the like. In such liquid crystal displays, on a rear side of a liquid crystal element, there is normally arranged an illumination device, namely a backlight (a planar light source device). The planar light source device serves as a mechanism for converting a linear light source such as a cold cathode discharge tube, for example, into surface light.
Specifically, a method in which the light source is arranged directly below the rear side of the liquid crystal element, and a method in which the light source is arranged to a side surface and an acrylic plate or other such translucent light guide pipe is used to convert the light to the surface form to thus obtain the surface light source (a side light method), are representative. On a light radiation surface there is arranged an optical element composed of a prism array or the like, and this serves as a mechanism for obtaining desired optical characteristics.
In the conventional planar light source device using the side light method, as shown in FIG. 44, on one side edge or a substrate which is the light guide pipe 1 and is composed of a translucent plate, there is arranged a linear light source 2 arranged such that it lies along an end surface 1a of the side edge portion, and a reflector 3 is attached so as to cover the light source 2. This serves as a mechanism in which direct light from the linear light source 2 and light reflected by the reflector 3 enter the light guide pipe 1, into an inner portion thereof, from a light incidence end surface which is the side edge portion surface 1a. 
One surface 1b of the light guide pipe 1 serves as a light radiation surface, and on this light radiation surface 1b there is arranged a light modulation sheet 5, on which is formed an array 4 in the form of substantially triangle prisms, of which top angles are directed toward a viewer. On the other hand, on a surface 1c of a side opposite from the light radiation surface 1b in the light guide pipe 1, there is provided a light extracting mechanism 6 on which there are formed multiple dots 6a, 6b, 6c . . . printed in a predetermined pattern using light scattering ink.
In the light guide pipe 1 to which the above light extracting mechanism 6 is formed, on the surface 1c side that is opposite from the light radiation surface 1b there is provided a reflection sheet 7 in close proximity to this surface 1c. Further, as a different example of this type of conventional planar light source device, as shown in FIG. 45, the light modulating sheet 5 has the array 4 of substantially triangle prisms formed thereon, which is disposed above the light radiation surface 1b with the top angles of the triangle prisms facing toward the light radiation surface 1b of the light guide pipe 1.
Further, a light extracting mechanism 8, which is provided to the surface 1c being on the opposite side of the light guide pipe 1 from the light radiation surface, is composed of multiple protruding portions 8a, 8b, 8c . . . each having its surface formed as a rough surface. The planar light source devices according to the above side light method are capable of taking greater advantage of lightness and thinness, which are general characteristics of liquid crystal display devices, so they are frequently used as the backlight in liquid crystal displays of portable personal computers and the like.
However, in the conventional transmission-type liquid crystal display devices, there was the problem that the construction was still rather complicated. The reason for this was that, particularly in the planar light source device, it was not possible to obtain an illumination optical system having a simple structure and being capable of achieving the desired optical characteristics, so the structure of the planar light source device had to be made complex. The resultant increased costs and the like have prevented this type of liquid crystal display device from being widely used.
That is, in the planar light source devices shown in FIG. 44 and FIG. 45 which were used as the backlight optical system in, say, the transmission-type liquid crystal display device, in order to make use of the illumination light from the planar light source device as effectively as possible, a prism sheet or other such optical sheet item was frequently used. Accordingly, the structure of the illumination optical system become complicated, and as a result, high costs were incurred due to difficult assembly as well as low yield.
The present invention was developed to solve the conventional problems; therefore, an object of the present invention is to provide an inexpensive planar light source device which is based on a novel, simply constructed illumination system that is superior in its efficiency of utilization of illumination light beams, uses a light reflection sheet that is a novel light guide pipe and novel light control member to thereby achieve effective utilization of light, and also has a structure that is simple and superior in easiness of assembly.
The present invention relates to a planar light source device, which is comprised as follows in order to solve the above-mentioned technical problem. That is, the planar light source device according to the present invention is comprising a light guide pipe formed as a plate (sheet) made of transparent synthetic resin with one surface thereof serving as a light radiation surface, and a light source arranged in the vicinity of at least one side edge portion of the light guide pipe; and light beams radiating out from the light radiation surface of the light guide pipe at the time when the light source is lit up has a peak of a distribution of radiation angles with respect to a right angle direction to the surface of the side edge portion of the light guide pipe to which the light source is arranged being within xc2x115xc2x0 from a normal line (perpendicular line) of the light radiation surface, and a half-width of the distribution of radiation angles being within xc2x130xc2x0.
Further, a planar light source device according to the present invention is comprising a light reflection sheet arranged to an opposite surface side from the light radiation surface of the light guide pipe; light beams that have entered the light guide pipe from the light source are radiated out from the opposite surface from the light radiation surface, and are reflected at the light reflection sheet; the reflected light is again made to enter the light guide pipe and is radiated out from the light radiation surface of the light guide pipe; and the radiated light at that time has a peak of the distribution of radiation angles with respect to the right angle direction to the surface of the side edge portion of the light guide pipe being within xc2x115xc2x0 from a normal line (perpendicular line) of the light radiation surface, and a half-width of the distribution of radiation angles being within xc2x130xc2x0.
Further, a planar light source device according to the present invention is comprising a light extracting mechanism provided to the opposite surface from the light radiation surface of the light guide pipe, and a light extracting mechanism makes a greater part of the light beams that have entered the light guide pipe from the light source radiate out from the opposite surface from the light radiation surface, and reflect at the light reflection sheet.
Furthermore, a planar light source device according to the present invention is comprising a light guide pipe with one surface thereof serving as a light radiation surface; a light extracting mechanism provided to the light guide pipe; a light source provided to a side edge portion of the light guide pipe; and a light reflection sheet arranged to an opposite surface side from the light radiation surface of the light guide pipe; the light extracting mechanism serves as a mechanism for radiating out, toward the light reflection sheet side, at least 65% or more of light beams radiated out from the light guide pipe; and the light reflection sheet is formed with an arrangement of multiple substantially identical and/or substantially similar base units composed of sloped reflection surfaces at a pitch of 5000 xcexcm or less.
In a planar light source device according to the present invention, which is provided with the above characteristics, it is preferable that, at least on one surface of the light guide pipe, there be provided corrugated unevenness having a pitch of 500 xcexcm or less, and, in this case, ridge lines of the corrugated unevenness be set in a direction that is substantially perpendicular to the light guide pipe side edge portion where the light source is arranged.
Also, in a planar light source device according to the present invention, it is preferable that the light extracting mechanism be formed by a pattern in which there are arranged multiple convex protrusions provided to the opposite surface side from the light radiation surface of the light guide pipe. Here, it is preferable that a value h/Wmin defined by a depth h of the convex protrusions and a minimum opening width Wmin, be 0.5 or greater.
Further, in a planar light source device according to the present invention, it is preferable that a coat layer made of a transparent insulating material be provided on the light reflection surface. In this case, it is preferable the metallic material be silver or aluminum, and the coat layer be composed of a transparent material having a resistivity of 1.0xc3x97166 xcexa9xc2x7cm or greater.
Furthermore, in a planar light source device according to the present invention it is preferable that a surface of the coat layer composed of a light-transmissive material which is provided on the light reflection surface be a flat and smooth surface. It is also preferable that a printed pattern be provided on the flat and smooth surface of the coat layer.